All-optical polarization modulator based on spatial soliton coupling

The possibility of realizing an all-optical polarization modulator is theoretically demonstrated. Basic device exploits the cross-phase modulation effect involving spatial solitons in a Kerr-like nonlinear material. It is shown that a weak wave (modulating signal) can be used to control a stronger wave (pump signal), in such a way to obtain a polarization switch, if their input phase difference is exactly equal to 90° and the two waves exhibit mutual orthogonal polarizations. Since the required relative phase may be difficult and sometimes impossible to maintain, an improved solution is then proposed which eliminates any effect due to random phase difference between the colliding solitons. It is based on the adoption of a suitable control device employing two additional nonlinear blocks excited by solitons with equal amplitude and linear polarization. The resulting field is then subjected to self-phase modulation. The structure is completed by a cascade of linear and nonlinear waveguides acting as proper soliton filters. This way, the behavior as an all-optical polarization modulator is ensured, even in the most critical input conditions, corresponding to a phase difference of 12.5° for our choice of the physical and geometrical parameters.